1. Field of Invention
The present invention relates to a radio frequency ripple zero crossing (RFZC) signal calibration method for an optical disk system. More particularly, the present invention relates to a radio frequency ripple zero crossing (RFZC) signal calibration method when an operating optical disk system switches from a track following operation to a track seeking operation.
2. Description of Related Art
In a modern computer system, optical disks are important storage media for holding a huge volume of data. To look for data within an optical disk, a pickup head needs to be properly positioned over a correct track. Moving speed and track crossing number of the pickup head is often controlled by reference signals including track error zero crossing (TEZC) signal and radio frequency ripple zero crossing (RFZC) signal. Hence, any error in the reference signals may lead to inaccurate movement of the pickup head and ultimately optical track-searching capacity.
In the past, radio frequency ripple zero crossing (RFZC) signal was produced by comparing the radio frequency ripple (RFRP) signal and the fixed radio frequency zero crossing (RFZC) slice level. That is, when RFRP is greater than the slice level, the RFZC value is 1. Conversely, if RFRP is smaller than the slice level, the RFZC value is 0. In a stable track-crossing condition, since the RFRP signal is relatively stable, the RFZC signal is also stable. However, there is minor distortion of the RFRP signal in the early stage of a transition from a track following operation to a track seeking operation. Therefore, duty cycle of the RFZC signal may deviate from the norm and the search capacity may deteriorate.F
FIG. 1 is a diagram showing the RFZC signal waveform for a track following to track seeking transition. FIGS. 2 and 3 are expanded versions regarding the early stage of the waveform diagram in FIG. 1 when the optical disk system switches from the following mode to the seeking mode. In FIGS. 1 to 3, numerals 110, 210 and 310 point to the RFRP signal waveforms. Numerals 120, 220 and 320 point to the RFZC slice levels. Numerals 130, 230 and 330 point to the resulting RFZC signals of comparisons between associated RFRP signals and the RFZC slice levels. Numerals 140, 240 and 340 indicate the status of the search flags that the search flags are pulled to low voltage levels within search control periods. In the early stage of the transition from the track following to track seeking operation, DC levels of the RFRP signals drift down slightly as shown in FIGS. 2 and 3. Hence, the low-level condition is maintained for 61.8 μs (Δt in FIG. 2) while the high-level status is maintained for 27.5 μs (Δt in FIG. 3) during the first cycle of the RFZC signal. In general, a cycle of such unbalanced magnitude (27.5/81.8) has considerable effect on the performance of an optical disk system.